25 September, 2020

Atmospheric CO2 varies across geologic time


  • CO2 ranged 180 - 4000 ppm in last 500 million years


  • CO2 changes impact living things via photosynthesis
    • primary source of energy in food chain
    • large evolutionary selection pressure


  • Large decline in CO2 last 30 million years


  • What does this mean for the current efficiency of C3 photosynthesis?

New pathways evolved ~30 MYA

Evolution of C4 photosynthesis





  • Calvin cycle moved to Bundle Sheath Cells surrounding leaf veins
    • chloroplasts now present



  • BSC cells are less permeable to gases
    • what does this mean for photorespiration?

C4 plants concentrate CO2 around Rubisco spatially



  • C4 mechanism separates Calvin cycle from high O2
    • 2 compartments


  • New enzyme, phosphoenolpyruvate carboxlylase (PEPC)
    • fixes CO2 to PEP (no affinity for O2)
    • new 4C malate pumped to bundle sheath cells
    • malate broken down to release CO2


  • Hyper efficient with use of CO2
    • How does this impact stomata behavior?

C4 is turbo-charged from of photosynthesis




  • Most productive C4 plants have rates ~50% higher than C3 plants
    • evolved independently ~70 times


  • ~435,000 plant species, only 2% are C4
    • 3 of top 10 crop plants are C4
    • extremely rare in trees (1 family)


  • C4 plants still account for 25% of productivity

Why is C4 photosynthesis not dominant?





  • Regeneration of PEP expensive (ATP)
    • in addition to ATP needed for Calvin cycle


  • Where is needed ATP generated?


  • Where should C4 plants live
    • efficiency vs dehydration

In hot conditions, the benefits of reduced photorespiration likely exceed the ATP cost of moving CO2 from the mesophyll cell to the bundle-sheath cell

C4 photosynthesis: Evolved independently 60-70 times

C4 vs C3 vs principles of evolution



  • Recruitment of enzymes into new functions


  • Massive shifts in distribution of parts
    • proteins and organelles


  • Anatomical modifications to cell structure
    • vein structure


  • Predict: In what type of plants would this most likely occur?

Why are there virtually no C4 trees?


  • Is it difficult for trees to transition from C4→C4?
    • long generation times


  • Why do C4 shrubs not evolve into C4 trees?
    • they should have improved growth…


  • Was shade a problem?
    • some Hawaiian C4 trees live in understory


  • C4 plants seem to have an issue growing tall…

Why C4 matters to you: Water-use-efficiency


  • Climate change impacts photosynthesis:
    • ↑ CO_2_
    • ↑ temperatures
    • variable precipitation
    • variations in humidity


  • C4 plants can make the same amount of sugars as C3, with less water usage
    • more CO_2_ assimilated per molecule of water loss
    • ability to keep stomata closed more often


  • C3 & C4 plants should both benefit from extra CO_2_
    • only works in C3 if enough H_2_0
    • huge advantage for C4 crops

Why C4 matters to you: Crops




  • C4 plants are economically important
    • corn, sugarcane, sorghum & switchgrass


  • So are C3 crops
    • beans, rice, wheat, potatoes (temperate crops)


  • Global water use is set to triple by 2050
    • 70% to the agriculture sector
    • 50% transpired through stomata

Can science TUNE photosynthesis?



  • Eliminating photorespirtion is unrealistic
    • can we tweak Rubisco?


  • Small improvement = huge gains in crop yield


  • 5% reduction = extra 68 million bushels of soybean
    • 23 million bushels of wheat
    • $540 million value (Walker et al. 2016)

Can science TUNE photosynthesis?

Genome work on C4 plants already underway

Special Topics lecture: C4 rice project